Process for filter aid production in alumina refineries

ABSTRACT

An improved process for the production of tricalcium aluminate (TCA) filter aid for use in an alumina refinery is described. Quicklime is slaked in a slaker tin using a suitable slaking solution to form a slaked lime slurry ( 10 ). A suitable surface-active agent is added to the slaking solution prior to slaking of the lime. Alternatively, the surface-active agents can be added to the slaked lime either during slaking or after slaking. The slaked lime slurry is then transferred to a stirred storage/transfer tank ( 12 ) before it is pumped to a lime aging tank ( 14 ). A concentrated Bayer liquor and steam are added to the tank ( 14 ) to provide a caustic aluminate solution that reacts with the slaked lime. Sufficient residence time is allowed in the lime aging tank ( 14 ) for the initial product of the reaction to “age” before use, forming relatively pure particles of the thermodynamically stable TCA. The addition of a suitable surface-active agent or agents at the appropriate concentration will significantly improve TCA formation in the lime aging facility. These improvements include a narrower and more symmetric sized distribution, and a more crystalline structure, leading to substantially improved filtration characteristics.

FIELD OF THE INVENTION

[0001] The present invention relates to an improved process for theproduction of a calcium aluminate filter aid with enhanced liquorfiltration characteristics for use within an alumina refinery.

BACKGROUND TO THE INVENTION

[0002] In most alumina refineries, bauxite is digested in a causticsolution under conditions of elevated temperature and pressure. Thisyields a slurry of mud in a concentrated sodium aluminate solution whichmust then be clarified to produce a solids-free liquor and a thickenedmud that is subsequently washed and discarded. The prevalent techniquefor slurry clarification involves allowing the solids to settle ingravity thickeners (or mud settlers), and decantation of the clarifiedliquor. The separation of the mud from the concentrated liquor isassisted with flocculants, whilst the “green” (or pregnant) liquor,which is free of all but the finest suspended solids, overflows from themud settlers. It is normal for the decanted liquor to then be furtherclarified by filtration, typically using pressure filters. Thisso-called “security” or “polishing” filtration step is critical inensuring that the pregnant liquor is free of suspended mud particlesthat would otherwise result in contamination of the product alumina.

[0003] Unaided, the cloths employed in these filters would blind veryquickly. This occurs because the fine suspended solids in the greenliquor become entrapped within the weave of the cloth, and then proceedto form a dense, highly resistive bed at the filter's surface.

[0004] To prevent this, it is common practice to supplement the feed tothe polishing filter with a filter aid, which acts to prevent clothblinding by the continuous formation of a bed of solids which trap themud particles whilst still allowing the free flow of liquor through theinterstices of the bed. An ideal filter aid will be cheap, chemicallyinert, and of such a size that the channels that form between the filteraid particles are just small enough to trap the mud particles, but notso small that they restrict the flow of liquid, or contribute toblinding of the filter cloth themselves. In most alumina refineries,this role is performed by tricalcium aluminate (also referred to as TCA,C3A or C3AH6).

[0005] TCA is chosen as it meets the above requirements tolerably well.It is cheap and relatively simple to produce, being formed through thereaction of lime (either as quicklime or, more usually, slaked lime)with caustic aluminate solutions. Generally, the production of TCA isperformed in concentrated liquors (either pregnant or spent) attemperatures of approximately 100° C., in tanks dedicated to thepurpose. The initial products of the reaction are calcium aluminatespecies of the C4A type (the most common form of which is also referredto as hydrocalumite). These intermediate calcium aluminate species arethermodynamically unstable under such conditions and possess relativelyhigh solubility products. Use of these intermediate species as filteraids is uncommon, because their comparatively high solubility can resultin calcium contamination of the product alumina. TCA, on the other hand,has an extremely low solubility, so its use does not result inappreciable contamination of the refinery's liquor stream. For thisreason, sufficient residence time is allowed in the reaction vessel forthe initial products of the reaction to “age” before use, formingrelatively pure particles of the thermodynamically stable tricalciumaluminate (TCA). The tanks in which the filter aid is produced aretherefore often referred to as “lime ageing” tanks

[0006] A serious drawback of the prior art technique for TCA filter aidproduction is that the distribution of TCA particle sizes is often verywide, and there is invariably a high proportion of very fine particles.This results in poor filtration rates and low filter cloth life, andnecessitates the use of a large number of filters to achieve adequatefiltrate flow.

[0007] Given the comparatively low solubility of calcium in Bayerliquors, it is reasonable to assume that reactions between calcium andthe aluminate ion occur at the particle surface, rather than via thedissolution of calcium hydroxide and subsequent re-crystallisation ofcalcium aluminate. Examination of the reaction of individual slaked limeparticles in Bayer liquors as a function of time, using scanningelectron microscopy and XRD analysis ¹suggests that this is indeed thecase. A surface layer of C4A-type material forms rapidly at the surfaceof the particle, while the core remains unreacted. The surface developsa characteristic reticulated appearance arising from the formation ofmany randomly oriented platelets of C4A. At this early stage, theparticle still retains the general form and size of the parent limeparticle,

[0008] As the reaction proceeds, the core of unreacted lime diminishes,suggesting either that calcium ions are diffusing outwards towards thesurface, or that aluminate ions are diffusing inwards. In addition, someof the surface C4A crystals begin to recrystallise into the familiaroctahedral TCA crystals. However, C4A and TCA have a substantiallydifferent density to calcium hydroxide and as the reaction proceeds,internal stresses are generated within the crystal. Cracks and fissuresdevelop within the particle, and as the reaction proceeds, the structurebegins to crumble. Given sufficient time, the particle will ultimatelydegrade into individual TCA crystals, each only a few microns in size.

[0009] The inventors believe that it is this mechanism that results inthe severely skewed, overly fine size distribution that ischaracteristic of TCA filter aid produced using the prior art technique.Given this mechanism, it is therefore not surprising that attempts toimprove filter aid morphology and size distribution by altering theconditions in the lime ageing tanks are not successful.

[0010] In the absence of any effective means to improve the sizedistribution of the TCA filter aid itself, a few processes have beenpublished which seek to alleviate the effect of poor quality filter aid.These prior art techniques invariably utilise flocculants, which serveto bind the decanted mud particles and fine TCA crystals into largerfloccules. For example, in U.S. Pat. No. 5,091,159 Connelly et al.describes the addition of dextran to the thickener overflow and filteraid to improve filtration performance. A similar approach for use inrefineries where sand filters are employed, rather than filter presses,is described in U.S. Pat. No. 5,716,530 by Strominger et al. Barham etal¹ describes the results of tests in which the filter aid issupplemented with a flocculant after it has been formed in lime ageing.However, these approaches are highly dependent upon the quality of thefilter aid and the amount of suspended mud, and results can be variable.Furthermore, the flocculant itself can seriously impair filtration,particularly if overdosed (as can occur readily in an attempt to controlan excursion in either TCA particle size or suspended mud).

[0011] The terms ‘A’ and ‘C’ used throughout this Specification refer tothe alumina and caustic concentrations of a Bayer liquor, as perconventional alumina industry parlance. Hence ‘A’ is the concentrationof sodium aluminate, expressed as the equivalent concentration of Al₂O₃,in g/L. The ‘C’ concentration is the sum of the sodium aluminate andsodium hydroxide concentrations, expressed as the equivalentconcentration of sodium carbonate, in g/L.

SUMMARY OF THE INVENTION

[0012] The present invention was developed with a view to providing animproved process for the production of a TCA filter aid for use inalumina refineries that is less susceptible to at least some of thedisadvantages of the prior art noted above.

[0013] According to the present invention there is provided a processfor the production of TCA filter aid for use in an alumina refinery, theprocess including the steps of:

[0014] slaking lime in a slaking solution to form a slaked lime slurry;

[0015] dosing said slaking solution or slaked lime slurry with asuitable surface-active agent;

[0016] reacting the dosed slaked lime slurry with a Bayer processliquor; and,

[0017] ageing the reaction products for a sufficiently long residencetime to permit substantially complete conversion of the reactionproducts to TCA.

[0018] The steps of dosing and slaking may occur simultaneously orsequentially.

[0019] Preferably quicklime is slaked to form the slaked lime slurry.Preferably the slaking solution contains some alkali. Typically theslaking solution has an alkalinity which falls within the range of 5 to30 g/l, expressed as equivalent grams per litre of sodium carbonate.

[0020] Preferably the surface-active agent is of a kind that adsorbsreadily at the surface of the lime particles during slaking. Suitablesurface-active agents include sugars, such as glucose or sucrose; and,polysaccharides such as starch. More preferably, the surface-activeagent is an anionic organic surfactant. Typically the anionic organicsurfactant is selected from the following group of compounds their saltsand derivatives: any anionic homopolymers or copolymers (e.g.polyacrylic acid and its co-polymers with acrylamide, or polymersbearing hydroxamate functional groups), hydroxamic acids, humic andtannic acids, sulphonated carboxylic acids, and various substituted monoand polycarboxylic acids, particularly polyhydroxy carboxylic acids.

[0021] Preferably a concentrated Bayer process liquor with high A/Cratio is employed. Preferably the flows of slaked lime slurry andconcentrated liquor are controlled so that the ratio of Al₂O₃ to CaOduring said ageing step exceeds 0.33:1, expressed as a molar ratio. Mostpreferably the ratio Al₂O₃ to CaO is within the range of approximately1:1 to 1.5:1, expressed as a molar ratio. Preferably the temperatureduring the ageing step is maintained at between 95° C. and 110° C., mostpreferably between 100° C. and 105° C. Typical residence time duringsaid ageing step is between 2 to 4 hours.

BRIEF DESCRIPTION OF DRAWINGS

[0022]FIG. 1 illustrates graphically the distribution of particle sizesin a TCA filter aid using the preferred process in accordance with thepresent invention, compared with TCA produced using the prior arttechnique; and,

[0023]FIG. 2 illustrates a prior art facility for the production of TCAin an alumina refinery in which the preferred process in accordance withthe present invention is performed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0024] The inventors have found that the physical characteristics of theTCA filter aid are very highly dependent upon the nature of the limeparticles from which it is formed. Thus, the geology of the limestoneand the conditions under which the limestone is calcined can greatlyaffect subsequent TCA quality. Unfortunately, these are parameters whichare rarely under the control of the Alumina refinery.

[0025] The present invention is based on the discovery that whenquicklime is slaked in a solution containing a suitable surface-activeagent or agents at the appropriate concentration, the resultant slakedlime slurry will improve TCA formation in the lime ageing facility.These improvements include a narrower and more symmetric sizedistribution, and a more crystalline structure, leading to substantiallyimproved filtration characteristic.

[0026] The inventors have found that, in general, improvements in thesize and morphology of TCA particles beyond that of the prior art cannotbe achieved through manipulation of the conditions in the lime ageingtank itself. For example, varying liquor composition, the amount oflime, temperature or agitation conditions gave no discernibleimprovement in the particle size distribution; on the contrary, manycombinations of these parameters can result in a substantial worseningof the particle size distribution.

[0027] Similarly, experimental evidence suggests that the addition ofsurface-active agents to the lime-ageing tank has little or no effect onthe size distribution or morphology of the TCA particles. Thesurface-active agents should preferably be added to the slaking solutionprior to slaking of the lime. Alternatively the surface-active agentscould be added to the slaked lime either during slaking or afterslaking.

[0028] A variety of surface-active agents could be employed for thepurpose described here.

[0029] Preferably the surface-active agent adsorbs readily at thesurface of the lime particle during slaking. Examples of surface-activeagents that could be used for this purpose include sugars such asglucose or sucrose, and polysaccharides such as starch. However, theinventors found that anionic organic surfactants are most effective. Anon-exclusive list of examples of this class of compound includes thefollowing materials, their salts and derivatives: any anionichomopolymers or copolymers (e.g. polyacrylic acid and its copolymerswith acrylamide, or polymers bearing hydroxamate functional groups),hydroxamic acids, humic and tannic acids, sulphonated carboxylic acids,and various substituted mono and polycarboxylic acids, particularlypolyhydroxy carboxylic acids.

[0030] The amount of the surface-active agent to be dosed is dependentupon the origin and reactivity of the quicklime used, the composition ofthe slaking solution, and the nature of the surface-active agent used.The requisite dose is best determined by experimentation, with thecorrect dose determined at the point at which the narrowest TCA sizedistribution is achieved, together with a suitably coarse mode particlesize. It should be noted that either too much or too littlesurface-active agent will result in a degradation of the filter aid'sparticle size distribution, though in neither case is this worse thanthe prior art.

[0031] The inventors have found that best performance is obtained if theslaking solution contains some alkali, as for example, if the slaking isperformed in the Alumina refinery's lake or process water. A suitablerange of alkalinity, expressed as equivalent grams per litre of sodiumcarbonate, is between 5 and 30 g/L. Lower concentrations can be used,but tend to produce undesirable quantities of coarse slaked limeparticles. Too high a concentration will result in the undesirableformation of calcium carbonate, unless the slaking solution is a puredilute sodium hydroxide solution.

[0032]FIG. 2 illustrates a typical prior art TCA production facilitythat includes a slaker 10 in which quicklime is slaked using a suitableslaking solution. The slaked lime slurry is then transferred to a siredstorage/transfer tank 12 before it is pumped to a lime ageing tank 14. Aconcentrated Bayer liquor and steam are added to the tank 14 to providea caustic aluminate solution that reacts with the slaked lime.Sufficient residence time is allowed in the lime ageing tank 14 for theinitial products of the reaction to “age” before use, forming relativelypure particles of the thermodynamically stable tricalcium aluminate(TCA).

[0033] Slaking can be performed in any suitable slaking system, with thesurface-active agent either added directly to the slaker (if in liquidform), or first dissolved or dispersed in the slaking solution. Apartfrom this, no special operating conditions are required other than toensure that complete slaking of the quicklime has occurred.

[0034] As indicated elsewhere, conditions within the lime ageingfacility are not overly critical, and acceptable performance will beobtained using the conditions and reactant concentrations used in theprior art procedure. However, best performance is obtained if thefollowing parameters are used:

[0035] 1) Concentrated liquors with high A/C ratios should be used, suchas the alumina refinery's settler overflow liquor.

[0036] 2) Flows of lime slurry and concentrated plant liquor should beadjusted such that the ratio of Al₂O₃ to CaO within the lime ageingtank, expressed as a molar ratio, is within the range of approximately1:1 to 1.5:1. In any event, the ratio of Al₂O₃ to CaO must always exceed0.33:1.

[0037] 3) The temperature in the lime ageing tank should preferably beheld at between 100° C. and 105° C. Temperatures lower than 95° C.should be avoided. If this is not done, C4A compounds may predominate,probably resulting in poor filtration and increased calciumcontamination in refinery product.

[0038] 4) The residence time should be between 2 and 4 hours. Lowerresidence times could result in incomplete conversion to TCA, whilelonger residence times will result in an increase in fine particleformation through attrition and decrepitation. Sensitivity to residencetime is increased if low A/C liquors are used.

[0039]FIG. 1 shows the particle size, expressed as a volume %distribution, for TCA filter aid produced by the Prior Art technique,together with the size distribution of TCA filter aid produced using thePresent Invention. As can be seen, the filter aid produced using theprocedure disclosed herein possesses a much narrower and moresymmetrical distribution, with a mode that is slightly coarser than thatof the Prior Art filter aid.

[0040] The following example illustrates one means of applying theinvention, and demonstrates the advantages of the new process.

Results of Plant Trial

[0041] To test the invention, a series of three full-scale trials wereconducted in the alumina refinery over a period of several weeks. Thesewere conducted as a sequence of on/off trials, alternating between theprior art procedure and the improved procedure. This enabled directcomparisons to be made between the two processes while partiallycompensating for variations in refinery operation.

[0042] Quicklime was slaked in the normal manner, but with the additionof a surface active agent, in this case sodium gluconate. The sodiumgluconate was delivered as a 400 g/L aqueous solution, directly into theslaking tank. Slaking was performed using process water, with the ‘S’concentration controlled to between 8 and 15 g/L, expressed asequivalent g/L of sodium carbonate. A slurry of 15% CaO by weight wastargeted. The sodium gluconate dose rate was calculated to give aconcentration of 390 mg/L in the slakers, or 110 mg/L in the lime ageingfacility.

[0043] After some time was allowed for the treated material to reach thelime ageing facility and for the TCA filter aid in the tank to be fullyturned over, samples were collected and compared with the normal TCAfilter aid. Typical results are shown in FIG. 1. After several days'operation, dosing of the sodium gluconate was ceased to allow thematerial to return to its normal state. This cycle of on/off dosing wasrepeated several times over the duration of the test work. In each case,the change in size distribution shown in FIG. 1 was achieved. Theperformance of the refinery's polishing (or security) filtration systemwas also monitored over this period. Performance of the filters in termsof the filtrate mass flux, measured as m/hr (or m³ of flow per hour, perm² of filter cloth), was recorded, with the filters operated at aconstant pressure of approximately 320 kPa.

[0044] Results of the on/off testing are shown in Table 1 below,together with the operating parameters for the test. TABLE 1 FiltrationPerformance During Plant Trial Filtration Rate Applied Pressure TrialNo. Test (m³m⁻²h⁻¹⁾ (kPa) 1 Prior Art 0.67 × 0.10 313 Present Invention0.94 × 0.05 304 2 Prior Art 0.73 × 0.08 328 Present Invention 0.91 ×0.06 325 3 Prior Art 0.67 × 0.08 335 Present Invention 0.91 × 0.06 331AVERAGE Prior Art 0.68 × 0.09 325 Present Invention 0.92 × 0.06 322

[0045] The average improvement in filtration rate is approximately 35%.Filtrate solids were typically less than 5 mg/L. In addition, during thetrial, calcia contamination of the refinery's product alumina fell toless than 0.027% w/w, from a previous average of approximately 0.038%.

[0046] Now that an example of the preferred embodiment of the process inaccordance with the invention has been described in detail, severaladvantages of the described process will be apparent. These include:

[0047] A. The process is simple to implement and should require no newequipment other than dosing facilities for the surface-active agent.

[0048] B. Dramatic and immediate improvements in filtration rate in thepolishing filtration facility of approximately 35% can be obtained. Thisreduces the number of filters that are required to achieve target flows,or alternatively, permits flows to be increased by up to 35% withoutrequiring additional filtration equipment.

[0049] C. Filtration performance is generally maintained for longerperiods, increasing the filter cycle time and reducing wash and cleaningrequirements.

[0050] D. The combination of B and C can reduce filter clothconsumption.

[0051] E. The process is robust. Small variations in the dosing of thesurface-active agent have little effect, while gross misdosing tends toresult in the TCA particle size distribution reverting to that of theprior art.

[0052] F. The improved consistency of the filter aid leads to reducedfiltrate solids, giving lower iron contamination of the product alumina.

[0053] G. The improved crystallinity and particle size of the filter aidleads to considerably reduced calcium contamination of the productalumina.

[0054] H. The improved consistency of the filter aid contributes to morestable operation of the polishing filtration facility. This can resultin an improvement in the refinery's availability factor, therebyincreasing productivity.

[0055] It will be apparent to persons skilled in the chemical andprocess engineering arts that numerous variations and modifications maybe made to the described process, in addition to those alreadydescribed, without departing from the basic inventive concepts. All suchvariations and modifications are to be considered within the scope ofthe present invention, the nature of which is to be determined from theforegoing description and the appended claims. Furthermore, thedescribed examples are provided for illustrative purposes only and arenot to be construed as limiting the scope of the invention in any way.

1. A process for the production of TCA filter aid for use in an aluminarefinery, the process including the steps of: slaking lime in a slakingsolution to form a slaked lime slurry; dosing said slaking solution orslaked lime slurry with a suitable surface-active agent; reacting thedosed slaked lime slurry with a Bayer process liquor, and, ageing thereaction products for a sufficiently long residence time to permitsubstantially complete conversion of the reaction products to TCA.
 2. Aprocess for the production of TCA filter aid as defined in claim 1,wherein the steps of dosing and slaking occur simultaneously orsequentially.
 3. A process for the production of TCA filter aid asdefined in claim 1, wherein quicklime is slaked to form the slaked limeslurry.
 4. A process for the production of TCA filter aid as defined inclaim 1, wherein preferably the slaking solution contains some alkali.5. A process for the production of TCA filter aid as defined in claim 4,wherein the slaking solution has an alkalinity which falls within therange of 5 to 30 g/l, expressed as equivalent grams per litre of sodiumcarbonate.
 6. A process for the production of TCA filter aid as definedin claim 1, wherein the surface-active agent is of a kind that adsorbsreadily at the surface of the lime particles during slaking.
 7. Aprocess for the production of TCA filter aid as defined in claim 6,wherein the surface-active agents are selected from the group consistingof sugars, such as glucose or sucrose; and, polysaccharides such asstarch.
 8. A process for the production of TCA filter aid as defined inclaim 6, wherein the surface-active agent is an anionic organicsurfactant
 9. A process for the production of TCA filter aid as definedin claim 8, wherein the anionic organic surfactant is selected from thefollowing group of compounds their salts and derivatives: any anionichomopolymers or copolymers (e.g. polyacrylic acid and its co-polymerswith acrylamide, or polymers bearing hydroxanate functional groups),hydroxamic acids, humic and tannic acids, sulphonated carboxylic acids,and various substituted mono and polycarboxylic acids, particularlypolyhydroxy carboxylic acids.
 10. A process for the production of TCAfilter aid as defined in claim 1, wherein a concentrated Bayer processliquor with a high A/C ratio is employed.
 11. A process for theproduction of TCA filter aid as defined in claim 10, wherein the flowsof slaked lime slurry and concentrated liquor are controlled so that theratio of Al₂O₃ to CaO during said ageing step exceeds 0.33:1, expressedas a molar ratio.
 12. A process for the production of TCA filter aid asdefined in claim 11, wherein the ratio Al₂O₃ to CaO is within the rangeof approximately 1:1 to 1.5:1, expressed as a molar ratio.
 13. A processfor the production of TCA filter aid as defined in claim 12, wherein thetemperature during the ageing step is maintained at between 95° C. and110° C.
 14. A process for the production of TCA filter aid as defined inclaim 13, wherein the temperature during the ageing step is maintainedbetween 100° C. and 105° C.
 15. A process for the production of TCAfilter aid as defined in claim 13, wherein residence time during saidageing step is between 2 to 4 hours.